In magneto-optics, a thin film of magnetic material is used to read, record, and delete data. The magnetic material accomplishes this by orienting the magnetization vectors within the material in one of two directions such as either an up or down. Depending on the selected direction, a label of either 1 or 0 is given to the respective magnetization vector. A sequence of these purposefully oriented vectors or bits is coded for specific digital information storage. To change or erase the data, the direction of one or more of the bits must be flipped. The magnetic force required to flip the magnetization from one configuration to the other, the coercive force, is temperature dependent and for a magneto-optical information storage system decreases with increasing temperature. To accomplish the read-write functions of the magneto-optical material an external bias magnetic field of magnitude less than the coercive force, at the system's ambient operating temperature, is applied; however, since the applied magnetic force is less than the coercive force no reorientation of the magnetization occurs. If, however, the temperature of this segment of the magnetic material is raised high enough the coercive force requirement decreases to the point where the bias field is now capable of flipping the orientation of the bit so as to align itself with the bias field vector. Since the objective of the magneto-optical system is to process large amounts of data rapidly, it is crucial that the system possess thermal stability and reversibility with respect to the coercive force value and the structural integrity as the area of interest cycles through the set temperature range required to flip the material's magnetization.
The characteristics of the thin magnetic film lend themselves for use as a recording medium for a laser based recording-retrieval system. For use with this system, a thin film of a magneto-optical medium coats a polycarbonate disk. The disk is then rotated and a small section, where information is to be entered, is subjected to an external bias field. If information is to be added or changed, this small section of the disk is subjected to illumination by a laser beam. The laser beam heats the section of the disk to a sufficient temperature to allow the magnetization of the thin film to align itself with the bias field. This permits the magneto-optical disk to record information in a manner similar to a magnetic tape.
The magneto-optical disk is read using a lower power setting for the laser and employing the Kerr effect. Under the Kerr effect, the plane of polarization of the laser as it interacts with the magnetized region of the thin film is rotated either clockwise or counterclockwise depending on the orientation of the magnetization in the thin film. This rotation permits the use of a polarizer to transform the rotation into a varying intensity, and thus, one can determine the binary value, 0 or 1, by measuring the difference in intensity of the reflected beam. Since the magnitude of the rotation is only of the order of 0.25 degree, the signal to noise ratio becomes an important factor in reading magneto-optic data and is a limiting factor in the magneto-optical material used on the disk. Signal to noise ratios for magneto-optical materials currently in use are in the 45 to 60 db range, Optical Disks Become Erasable, Robert Freese, IEEE Spectrum, Feb. 1988, p 43.
The magneto-optic information storage technology provides a more stable and a more densely packed medium than a magnetic recording medium. Due to the high coercive force at room temperature, it is very difficult to inadvertently erase data from the magneto-optical disk as compared to the magnetic disk which can experience data loss from the magnetic field associated with 600 volt power source. The most advanced Winchester disks have a magnetic recording density of 43 megabits per square inch while the current magneto-optical disk has a density of 300 megabits per square inch, Optical Disks Become Erasable, Robert Freese, IEEE Spectrum, Feb. 1988, pp 41-45.
In developing a magneto-optical medium for use with optical disks, several factors are important for optimal storage of information. To optimize data storage in the smallest physical space, the magnetization associated with the magneto-optical material should be oriented perpendicular to the plane of the medium. This is referred to as a perpendicular or vertical medium as opposed to a longitudinal medium which has its magnetic orientation in the plane of the medium. Perpendicular media have the energetic advantage of lower demagnetizing forces on opposing bits than longitudinal media. As was mentioned previously, temperature stability and signal to noise ratio also play critical roles.
The ultrathin regime refers to the thickness range that is shorter than the penetration depth of the light source. The ultrathin regime is distinctly different than the thick-film regime and possesses unique magneto-optical properties. The ultrathin regime exhibits characteristics of the Faraday effect because the light transmitted through the magnetic film reflects from the surface of the metallic substrate and travels back through the magnetic film a second time. The larger the reflectivity mismatch at the solid-solid interface, the larger the response from the Faraday signal. There are also ways of enhancing the Kerr rotation which involve the reflectivity minima associated with the rotation maxima. However, since the signal-to-noise ratio of a device will depend on the total signal strength, it is not optimal to enhance the rotation at the expense of the reflectivity.
The technique of ultrahigh vacuum and molecular beam epitaxy can be used to fabricate the ultrathin film on the substrate. Such films are grown by epitaxial deposition of magnetic material onto selected non-magnetic single crystal substrates. To achieve the thicknesses of interest, iron films are grown epitaxially layer-by-layer and take on the substrate inplane lattice spacing.
Ultrathin magnetic films offer the possibility of stabilizing the desired perpendicular orientations of the magnetization. It is known that surfaces and interfaces of ultrathin magnetic films possess physical properties that can dramatically differ from those of similar bulk materials. Reduced coordination and the disruption of the translational symmetry of the material alter their electronic, vibrational, and magnetic properties. A subtle manifestation of these changes in environment is that the spin-orbit coupling that governs magnetic anisotropy can energetically favor perpendicular spin orientations in the surface region of materials whose bulk symmetry is cubic. As the magnetic-layer thickness increases, the demagnetizing-field term overwhelms the surface contribution and the film reverts to a longitudinal magnetization. This suggests that as the thickness of an ultrathin medium is increased there is a region where the magnetization vector changes orientation from perpendicular to longitudinal based on thickness alone.
Accordingly, it is an object of this invention to provide a nonvolatile, high density, high signal to noise ratio, erasable medium for data storage using a ultrathin film of magnetic material epitaxially deposited on a non-magnetic substrate.
It is still a further object of this invention to provide a magnetic storage medium that utilizes the properties of an ultrathin film to maintain a vertical magnetization of the medium.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.